Adding Circle CI to master

[maxsam4]

TODO

1. Enable CircleCI
2. Set coveralls repo token in CircleCI environment
3. Set Github login username and password in CircleCI environment for pushing docs
4. Disable PR builds in Travis as coverage is now handled by CircleCI

Main changes in the CI workflow:

1. Travis is used for tests
2. CircleCI is used for coverage and docs
3. Docs are only generated when a commit is pushed to master branch
4. Coverage is now generated for every commit rather than only PR commits

I have also configured our test script to support parallelism offered by CircleCI. i.e, if in future we move tests to CircleCI from Travis, we will be able to split tests into X simultaneous parts for faster execution. Timing data of various configs available on Asana.